Events accompanying the onset of immunoglobulin synthesis in the African Clawed Frog, Xenopus laevis, are being investigated. Xenopus demonstrates a precocious onset of immunoglobulin Synthesis with IgM first being detected 48 hours after fertilization. We have used recombinant DNA technology to produce cDNA plasmids containing Xenopus immunoglobulin heavy chain sequences. Hybridization experiments using these sequences as a probe have demonstrated that immunoglobulin mRNA synthesis begins only 24 hours after fertilization. Research in progress currently involves attempts to isolate heavy chain genomic sequences from libraries of germline and lymphocyte Xenopus DNA in lambda bacteriophage. The structures and DNA sequences of these genes will be compared with each other and with linoun mammalian sequences to attempt to understand the evolution of these genes and the structural basis for gene rearrangement. In addition to these studies we are currently examining the structure and properties of the heavy chain RNA present early in development to try to understand whether it is being used as a messenger RNA.